In the context of passive transport (diffusion and osmosis), which process requires no energy input (ATP) from the cell?

• A. Simple diffusion of oxygen across the phospholipid bilayer
• B. Endocytosis of extracellular fluid by pinocytosis
• C. Exocytosis of neurotransmitters from synaptic vesicles
• D. Primary active transport of ${\mathrm{Na}}_{+}$ and $K_{+}$ by the sodium-potassium pump

Answer: (A)